Formation resistivity measurement while drilling, utilizing physical conditions representative of the signals from a toroidal coil located adjacent the drilling bit



Oct. 29, 1968 PREDENTATIVE OF THE SIGNALS FROM A TOROIDAL COIL LOCATED ADJACENT THE DRILLING BIT Filed July 29, 1963 5 SheetsSheet 1 3,408,561 TILIZING 5 FROM ets-Sheet 5 N IN 5 LLIN SIG ILL ET AL HILE DRI OF THE NT THE DR H. RED INE Y MEASUREMENT W CONDITIONS REPRESENTATIVE IDAL COIL- LOCATED ADJACE F. ESISTIVIT ION R ICAL Oct. 29, 1968 FORMAT PHYS A TORO Filed July 29, 1965 F. H. REDWINE ET AL VITY ME ASUREMEN A TOROIDAL C Filed July 29, 1963 5 Sheets-Sheet 4 FIG. 8

$335 3 on: muqmmam FIG. 7

RESENTATIVE O A T F THE SIGNALS FROM OROIDAL COIL LOCATED ADJACENT Filed July 29, 1963 THE DRILLING BIT 5 Sheets-Sheet 5 200 209 AMF. I2! 2!! 12V 2o5 7 i E 220 40 T -2lo 208 INTEGRA TOR run I PULSE DIVIDER-fl25 259 a 26 2 s f 270 2 PULSE VALVE SOLENOID 27 United States Patent CATED ADJACENT THE DRILLING BIT Fletcher H. Redwine, Irving, and William F. Osborn, Dallas, Tex., assignors to Arps Corporation, Garland, Tex., a corporation of Delaware Filed July 29, 1963, Ser. No. 298,298 14 Claims. (Cl. 3246) This invention relates to Well logging and has particular application to systems for determining properties or characteristics of earth formations in a well while such a well is being drilled.

Various systems have been proposed or have been employed in the past for logging a well to determine one or methods of logging while drilling utilize a number of spaced electrodes placed near the lower end but insulated has been electrically insulated from the rest of the drill pipe so that current may be passed between the two electrically separate elements. Flow of such current is modified by the electrical resistivity of the earth formation near the drill bit.

Such systems share certain common disadvantages including, for example, the necessity for insulating electrically a section of the drill collar from the rest of the drill string. During the process of drilling, the whole string is constantly being subjected to violent shocks in tension, compression and shear which must be transmitted mechanically through the insulation. Since insulating materials are relatively weak and more susceptible to fracture and fatigue than steel pipe, all such systems mechanically are unreliable in addition to being expensive to manufacture and maintain. Likewise, such systems share the disadvantage that the insulation, which is on the outside as well as on the inside of the drill collar, is exposed to abrasion and erosion by the drilling fluid, making frequent and costly replacements of such insulation necessary.

In another system, such as disclosed in US. Patent No. 2,354,887, it was proposed to mount a toroid shaped coil of wire, which is wound on a core of ferro-magnetic material which encircles the drill collar near the drill bit, in such a way that the axis of the toroid is parallel to and preferably coincides with the axis of the drill collar. It was expected that the impedance of such a coil would be affected sufiiciently by the impedance of the electric path of its one-turn secondary winding consisting of the drill pipe, the formation near the drill bit and its return path through the earth, to produce a usable measurement of formation resistance at the drill bit. However, it has been found that the variations in the electrical resistance of the formation near the bit in the normal range of such values do not alter the impedance of the coil sufliciently to make meaningful measurements possible. It is the purpose of the present invention to overcome difliculties, such as the foregoing, through the use of a separate de tector coil, preferably near the bit. The voltage induced in such detector coil essentially will be proportional to the conductivity of the formation near the drill bit.

It is a further object of this invention to provide a section of pipe between a transmitter coil and a detector coil through which part of the current will pass into the formation in a predominantly horizontal direction, thus forcing the remainder of the current through the detector coil and the drill bit at the contact between the drill bit and the bottom of the borehole.

In accordance with the present invention, there is provided a transmitting toroid mounted on the outer wall of a drill stem at a point spaced above a drill bit. A receiving toroid is mounted on the drill stem preferably immediately adjacent to the bit. An alternating current mitting toroid for induction of alternating current flow longitudinally through the bit to the formation at the bit-formation contact. Means connected to the receiving toroid translate induced voltages into pressure pulsations in the mud stream flowing through the drill stem for telemetering resistivity information to the earths surface.

In a preferred embodiment, the transmitter and receiver are mounted at spaced points along a drill sub adjacent a drill bit. The transmitter and receiver are each mounted in insulating bodies which encircle the sub with protective shrouds extending over both the transmitter and the receiver with a non-conductive zone in the surface of the sub to promote current flow through the bit to the earths formation.

For a more complete understanding of the present invention and for further objects and advantages thereof, reference may now be had to the following description taken in conjunction with the accompanying drawings in which:

FIGURE 1 is an borehole in section;

FIGURE 2 schematically cuit employed in FIGURE 1;

FIGURES 3A-3C are sectional views of a sensing sub constructed in accordance with the invention;

FIGURE 4 is a sectional view taken along line 44 of FIGURE 3B;

FIGURE 5 shows the electrical network of the invention in block form;

FIGURE 6 illustrates time pulse modulation employed in the invention;

FIGURE 7 is a modification of the system of FIGURE 3B employing an insulating spacer;

FIGURE 8 is a further modication in which a transmitter and receiver are closely coupled; and

FIGURE 9 is a circuit diagram of a down hole portion of the system.

In FIGURE 1, a drilling systemis shown in which a drill stem 10 is driven by a rotary table 11 for penetration of a formation 12 by a bit 13. The bit 13 is connected to the drill stem 10 by a sub 20. The drill stem is supported by conventional draw works in a derrick 15. Drilling mud from -a slush pit 16 is forced through the drill stem 10 by .a mud pump 17 and passes through a detector 18 and a hose leading to a swivel 19. It is the purpose of the present invention to provide for continuous measurement at the earths surface of the resistivity of the formation 12 including formations as they are newly exposed by the bit 13, The measurement of the formation resistivity prior to invasion by drilling fluids is highly desirable in the interpretation of the stratigraphy through which the resultant borehole 21 passes and, thus, the successful measurement of resistivity as drilling progresses is highly valuable.

In accordance with the present invention, pressure pulses are generated in the mud stream flowing down through the drill stem 10 by a mechanism contained in sub 20. The repetition rate of the pressure pulses transmitted to the surface hereinafter be described, the pulses being sensed by detector 18 and recorded as a function of time or as a function of the depth of the bit 13. A continuous log may thus be produced during the drilling operation, The instrumentalities employed for utilization of the energy of the mud stream for production of the pressure pulses elevational view of a well showing the illustrates the measuring cir- Representative of prior art methin general are known. ods and systems for telemetering by Way of the mud streams are Patent Nos. 2,524,031, 2,658,284 and 2,659,- 046 to J. J. Arps and Patent No. 2,898,088 to Alder.

FIGURE 2 schematically represents operations of the present invention wherein the sub 20, shown dotted, is coupled at the lower end thereof to the bit 13. A transmitting toroid 50 encircles the sub at a point spaced above the bit 13. A receiving toroid 40 encircles the sub 20 immediately adjacent to the bit 13. Current flow along a path represented by the loop 80 is established by flow of alternating current in the winding of the transmitter 50. Since the sub 20 is conductive, some of the current may leak from the conductive body of the sub to the adjacent formations along a path, the resistivity of which is represented in FIGURE 2 by the legend Rg. The path from the sub 20 to the formations, which includes the bit 13, will be of resistance Rb which will vary in dependence upon the resistivity of the newly exposed formation. Thus, the formation resistivity Rb of FIGURE 2 will control the flow of current through receiver 40. It is the variations of resistivity Rb which are of primary interest in the present invention. Variations in the current linked to the receiver 40 induce voltages in receiver 40 which appear at the output terminals of the winding and which will be employed for sending impulses to the earths surface which are representative of such variations.

One embodiment of the system of the present invention is illustrated in FIGURES 3A-3C. Where consistent, like parts have been given the same reference in FIG- URES 3A-3C as in FIGURES l and 2.

Sub 20, in FIGURES 3A and 313, comprises a central mandrel 22 having a central flow channel 23 extending downwardly to the threaded coupling leading to hit 13. The threaded pin of bit 13 is served into the box 24. The pin on the upper end of the mandrel 22 is threadedly secured to an upper section 25 and has a cylindrically re-entrant opening 27 to receive the lower end of a flow crossover element 28 which extends upward into an instrument housing in the upper section 25.

Mandrel 22 of sub 20 is a heavy-walled steel drill collar, reduced in diameter from its upper shoulder 30 t0 the lower shoulder 31 adjacent to the coupling 24. A short insulating cylinder 42 of insulating character is mounted at the lower end of sub 20. The outside diameter of the cylinder 42 corresponds generally with the maximum diameter of the sub 20. Receiver is embedded in cylinder 42. Receiver 40 is a detecting coil for measurement of formation resistivity. A steel band 43 has a cylindrical extension which enshrouds the receiver 40, protecting the same against abrasion. The outside diameter of the band 43 preferably corresponds with the diameter of the sub 20.

A spacer cylinder 45 is telescoped onto the mandrel 22 and is secured thereto by suitable studs 55a extending through the cylinder 45. A transmitter is embedded in a second short insulating cylinder 52 which is positioned on top of an insulating cylinder 54. A steel spacer 53 is positioned above the insulating cylinder 52 and includes a cylindrical extension which enshrouds the transmitter 50. A pair of steel rings 52a are mounted in the lower end of the insulating cylinder 52. A plurality of steel rings 54a are mounted in the insulating cylinder 54. Rings 52a and 54a are of diameter corresponding with the outer diameter of the sub 20. They are cast in insulating material, such as an epoxy resin, so that they will provide protection against abrasion while being maintained electrically insulated one from the other and from mandrel 22. The ring 54 initially is cast on a metallic supporting cylinder 54b. The cylinder 54b encircles the reduced diameter portion of the mandrel 22. It Will be noted that the transmitting toroid is embedded within the insulating material of the cylinder 52. A short spacer cylinder 55 is then secured to the mandrel 22 above the transmitting toroid 50 with the upper end of cylinder 55 coinciding with shoulder 30.

As illustrated in the sectional view of FIGURE 4, the cylinder 55 is secured to the mandrel 22 by four studs 55a. Cylinder 45 is similarly secured by studs 55a to mandrel 22. The spacers 45 and 55 provide a measure of mechanical shielding for the non-conductive cylinders 42 and 52 and, along with the bands 43 and 53, provide protection for the receiver 40 and the transmitter 50. A ring 39 of high magnetic reluctance such as copper is mounted beneath the toroid 40. Ring 39 provides a measure of magnetic shielding for receiver 40.

In the embodiment illustrated in FIGURES 3A-3C, a circuit is provided which includes the transmitter 50. Connections are completed by way of a plug unit 60 and a cable 61. The plug unit 60 is positioned in a recess 1n the inner wall of the spacer 53. The mandrel 22 has a groove 62 in the wall thereof extending from a hole or channel, to a point adjacent to the upper end of the plug unit 60. One terminal of the coil of transmitter 50 is thus electrically connected through cable 61 to a plug 70 mounted in the upper end of the flow crossover unit 28. The other terminal of transmitter 50 is connected to the sub which is at ground potential. The crossover unit 28 has an isolation channel for passage therethrough of the cable 61 and is secured to the upper end of male coupling 29 by means such as suitable set screws, not shown. The plug 70 mates with a plug which is located in a recess 76 in the bottom of an instrument housing 77.

Plug 75 is one of four plugs mounted in the recess 86 in the bottom end of a high pressure bulkhead 78. Bulkhead 78 is threaded into the lower end of the instrument housing 77. Four conductive rings, such as ring 79, are mounted one above the other to form a cylindrical extenison of the bulkhead 78. As illustrated. the plug 75 is connected by way of a wire 79a to the upper ring 79. The ring 79 has a resilient, inwardly directed contact 79b which is adapted to make contact with a conductive segment on a male insert, not shown, to be centrally located within the housing 77. Ring 79 and the remaining three rings are insulated from each other and from the housing in which they are fitted. It will be noted that the plug at the end of cable 85 is connected to the second ring 87 by a suitable wire by way of plug 88. The remaining two plugs in the recess 76 serve to complete circuits leading to ground. Thus, the plug 70 completes the ungrounded side of the circuit for the transmitter 50. The plug 88 serves to complete the ungrounded side of the circuit for the receiver 40. The transmitter 50 is energized from an AC source in housing 77 by way of plug 75. The detected signal from receiver 40 is applied to the instrumentation in housing 77 by way of plug 88.

When energized, transmitter 50 produces a magnetic field. Electrical currents are induced in the sub 22 which is a part of the single-turn current loop coupled to the transmitter.

The loop 80 is diagrammatically represented in FIG- URES 3A and 3B by the dashed line. The loop 80 includes the portion of the current path flowing in the mandrel 22 and the drill bit 13. Primary flow between the drill stem and the formation below the transmitter is through the teeth of the drill bit to the newly exposed formations over which the teeth ride. Current may leak directly to the borehole wall from the spacer 45 as indicated by arrows 81. Most of the leakage current passes through the mud column from the spacer 45 and th through the formation.

The current loop is closed above the transmitter 50 by return flow to the sub 20 and to the drill stern above the sub. The leakage current represented by arrows 81 acts as a current sheath or guard so that current flowing from bit 13 is forced to return through the earth well beyond the wall of the borehole.

Since the resistance through the earth back to the drill stem and through the drill stern itself is very low compared with the resistance at the bit, measurement of bit current is essentially a function of the formation resistance at the bit. I

As illustrated in'FIGURE 3A, the receiver 40 is positioned as close as possible to the bit 13 so that substantially all of the current linked to the receiver 40 passes into the formation at thebit contact. The progressive exposure of new formations by the bit is accompanied by the measurement of resistivity since, as above noted, the primary variable in the current loop 80 is represented by the bit-formation contact. The flow of current through the section of the mandrel 22 and the spacer 45 between transmitter 50 and receiver 40 effectively forms a current guard permitting a more precise measurement of uninvaded formation resistivity by reason of the limited linkage to receiver 40.

Arrows 81 represent the flow of current from the sub 20. In a preferred embodiment, the outer wall of the sub 20 below the receiver 40 is covered with an insulating sleeve82. Sleeve 82 prevents current linked to the receiver from flowing through points other than the bitformation contact. Thus a guard above the receiver forces the bit current to travel in the formation well behind the borehole wall. The sleeve 82 prevents flow of receiver-coupled current except through the bit-formation contact. 1

Electrically the system may be represented by the equivalent circuit shown in FIGURE 5. The driving voltage is applied by way of cable 61 to the transmitter 50 at input terminals 100. The condenser 101 represents Further, as to the transmitter 50, inductance 103 is the leakage inductance; inductance .104 is the primary inductance; resistance 105 is the equivalent core loss resistance; and inductance 106 is the secondary leakage inductance.

Resistance 107 is the resistance in the path of the guard current. Resistance 108 is the apparent formation resistance in the loop 80.

As to receiver 40, the inductance 109 is the primary leakage inductance; inductance 110 is the primary inductance; inductance 111 is the secondary inductance; and capacitance 112 is the shunt capacitance.

The voltage induced in receiver 40 appears at output terminals .113. The latter terminals include a plug unit 83 which is shown in FIGURE 3A. A cable extends upwardly through a channel 84 and through a port 85 cordance with the bit-formation resistance.

In a preferred mode of operation, the voltage repremitter and receiver relatively small.

FIGURE 5, in block form, illustrates the in the receiver 40. An alternating current oscillator 120 is connected to the input terminal 100 of the transmitter 50. The output terminals 113 of the receiver 40 are connected to a high impedance amplifier 121 whose output is applied to a rectifier 122. The rectified voltage is then applied to a frequency converter 123 to produce output pulses at a rate proportional to the magnitude of the voltage from rectifier 122. The output of the converter 123 is applied by way of an amplifier 124 to a dividing network 125 whose output is a low-frequency, time-spaced pulse train.

The time interval between pulses is to be measured at the earths surface. For this the pulse divider 125 is applied to a monostable multifunction of depth.

An embodiment of a valving system in simplified form is shown in FIGURE 3C. The instrument housing 77 includes the electrical units 121128 of FIGURE 5 with the solenoid 127 adapted to actuate a shaft 130 which supports a valve element 131. The valve element 131 extends into an orifice in a spout 132. By varying the position of the valve element 131, the impedance to fluid flow, as represented by the arrows 133, may be changed. Abrupt variations in the How impedance are propagated up the mud stream through the central channel 134 to the earths surface where they are detected. A screen unit 135 is positioned at the lower end of the channel 134 and above the valve unit 131 to minimize the possibility of clogging the valve in its operation.

Only a portion of the valve system is shown in FIG- URE 3C. Included is an upstream bladder 136 positioned above the valve element 131. The instrument housing 77 included a hydraulic drive system for shaft 130 which is responsive to actuation of the valve solenoid 137. In addition, a downstream bladder (not shown) is employed to sense downstream pressure. Control of the valve element 131 is accomplished through the use of the bladder 136 and the other elements above mentioned to convert signals applied to terminals 113, representative of resistivity, into pressure pulses. A suitable valving system is illustrated and described in Patent No. 2,898,088 to Alder. In a preferred embodiment, the valving system is actuated primarily by energy in the mud stream with the solenoid 127 serving merely to control the valve action at low power level. For this purpose, a chamber 137 is provided behind the bladder 136 with the wall inside the bladder 136 being perforated in order to introduce pressure variations into the chamber 137. The operation is such that the valve element 131 changes from one position to another for predetermined time intervals to introduce pressure variations in the mud stream which are of square wave character. Since the system operates in response to energy from the mud stream and involves a hydraulic system which requires recovery time, an inhibit-delay unit 128 is connected between the output of the multivibrator 126 and the output of the pulse divider 125 to prevent the divider 125 from responding to input pulses thereto. A fixed delay is imposed on the system by unit 128 to permit the valve system to recover following each pulse. The delay, thus introduced, is compensated at the surface so that the rate of pulses appearing at the surface when modified to accommodate the delay is representative of resistivity at the bit-formation contact.

In accordance with the invention thus far described, alternating current is spaced a substantial distance from the receiver 40. Receiver 40 is preferably mounted as close to the bit as is possible in order that all of the current linked to the receiver 40 will flow through the bit-formation con-' tact with a minimum of leakage. The spacer 45 is employed to enhance to a degree leakage of current above the receiver 40.

In contrast with the foregoing, a modification is illustrated in FIGURE 7 where the receiver 40 is spaced from the transmitter 50 in the same manner as in FIG- URES 3A-3C, except for the fact that the spacer positioned between them and identified as spacer 150 is of insulating character, thus preventing the leakage of any current in that portion of the sub 20 between the transmitter 50 and the receiver 40. In all other respects, the embodiment of FIGURE 7 is the same as the embodiment of FIGURES 3A-3C.

A further embodiment of the invention is illustrated in FIGURE 8, wherein the transmitter 50 and the receiver 40 are mounted immediately adjacent to one another so that a close, tight coupling factor is effective therebetween. In both FIGURES 7 and 8, the receiver 40 is shielded by a band 43'. The transmitter 50 is shielded by a band 53'. In FIGURE 8, transmitter 50 and receiver 40 are illustrated near the upper end of the sub 20. In practice, they may be located near the bit or may be located at a point substantially above the bit, depending upon the particular measurement for which the system is to be employed. In FIGURES 7 and 8, much of the detail has been omitted because of the identity thereof with the structure of FIGURES 3A-3C.

FIGURE 9 illustrates, in detail, the circuit diagram which is shown in block form in FIGURE 5. While not shown, a power oscillator is employed to energize the transmitting toroid. In one embodiment of the invention, the transmitter was operated at a level of about 25 milliwatts at a frequency of 500 cycles per second. The input impedance to the transmitter 50 was about 5000 ohms. The transmitter 50 is thus linked by a current loop to the receiver 40.

It will be noted that the receiver 40 is connected at one terminal to the positive terminal 200 of a battery 201. The negative terminal of battery 201 is connected to ground. The other terminal of receiver 40 is connected by way of a condenser 202 and resistor 203 to the input of an amplifier 121 having three emitter-follower stages 205, 206 and 207, which feed a transformer 208 at the output of follower 207. The amplifier 121 is supplied from battery terminal 200 by way of a resistance 209 and a regulator 210 comprised of Zener diodes which maintain the bus 211 at a potential of 12 volts above ground. Thus regulated, the amplifier 121 provides a gain of about as between the input and the voltage from the transformer 208.

A condenser 212 is connected in parallel with the receiver 40 to tune receiver 40 to the frequency of the oscillator 120.

The secondary winding of transformer 208 is connected by way of condenser 220 to the second section of amplifier 121. The second section includes three emitterfollower stages 222, 223 and 224. The output stage 224 feeds an output transformer 225. The output section is supplied from a 16 volt bus 226 which is fed from bat tery 201. A higher supply voltage is employed in the output section since the signal amplitude is somewhat greater than that which must be accommodated in the first section of amplifier 121.

The signal from the secondary winding of transformer 225 is rectified in section 122 by the diode 230. The voltage thus developed is applied to condenser 231. The rectified signal voltage thus appearing across condenser 231 is employed in section 123 to produce output pulses which vary in their repetition rate in proportion to the magnitude of the voltage on condenser 231. The latter pulses appear on the output bus 241 and are produced through the cooperative action of a differential amplifier circuit involving amplifier 242 and an integrated amplifier circuit involving the amplifier 243.

The circuit leading from condenser 231 includes a voltage dividing network formed from resistors 232 and 233. The juncture between resistors 232 and 233 is connected to input terminal a on the differential amplifier 242. The resistor 233 is adjustable. The lower terminal of condenser 231 is connected by way of an RC biased network including condenser 234 and resistor 235, which are connected in parallel, to a reduced voltage bus 236. The lower terminal of condenser 231 is also connected by way of resistor 237 to the l6-volt bus 226.

Amplifiers 242 and 243 in one form of the invention were Philbrick amplifiers Model PP65. The amplifier 242 serves as a differential amplifier, producing an output pulse each time the input voltage applied to the terminal b coincides with the voltage applied to terminal a. Amplifiers of the type above identified normally work with the operating voltages on the intermediate terminals at +15 volts, 0 volts and -l5 volts, respectively. In the circuit illustrated in FIGURE 9, the upper terminal is connected to the l6-volt bus 226. The center terminal is connected to an 8-volt bus 259. The bottom terminal is connected to ground. Trimming resistors, such as resistor 258, are provided to adjust the amplifier such that the output on bus 241 is balanced when terminals a and b are connected together.

Conduction in amplifier 242 is switched by application thereto by a control signal which is developed at terminal 250 by an integrator which embodies amplifier 243. The integrator embodies condenser 251. Condenser 251 is connected at one terminal to the 8-volt bus 236 by way of the voltage dividing network including resistors 252 and 253. The juncture between resistors 252 and 253 is the output terminal of amplifier 243. The second terminal of condenser 251 is connected by way of variable resistor 254 to a line 255. Line 255 leads to a mid-tap on a voltage divider comprising resistors 256 and 257 which are connected in series between 16-volt bus 226 and the 8-v0lt bus 236. The juncture between condenser 251 and resistor 254 is connected to the b input terminal of amplifier 243. The a input terminal is connected to the 8-volt bus 236.

In operation, the network including condenser 234 and resistor 235 serves to bias the output voltage from condenser 231 to 13 volts when the resistivity of the formations is very high such that zero voltage is induced in receiver 40. Further, the voltage terminal 250, normally at the start of each integrator cycle is at 13 volts. When the resistivity of the formations is of magnitude other than approaching infinity, there will be a time lag introduced by the operation of the integrator before the voltage at terminal 250 drops to the voltage equal to the signal voltage at terminal a of amplifier 242. The integrator causes the voltage at terminal 250 to decrease linearly with time. At the instant that the integrator voltage equals the signal voltage, the amplifier 242 shifts in conduction, producing a positive voltage change at the output. This voltage output is then applied to a monostable multivibrator 260, the output of which appears at terminal 261. A reset circuit including transistor 262 is connected by way of conductor 263 to the b terminal of the amplifier 243. The integrator is reset to carry out repeated comparison cycles.

A coupling circuit, including transistor 270, is connected by way of condenser 271 and a diode 272 to a pulse amplifier 124 whose output drives a five stage divider 125. Divider 125 is made up of five bistable multivibrators, 276, 277, 278, 279 and 280. Divider 125, thus formed, produces one pulse on output line 282 for every 32 pulses appearing on the input line 241.

The output line 282 is connected to a monostable multivibrator 126. The monostable multivibrator 126 is connected to the valve operating solenoid 127 so that the 9 131 of FIGURE 3C Will be pulsed once pulses generated by the conversion cirvalve element for every 32 cuit 123.

The output voltage from the multivibrator 126 is applied by way of diode 286 to a monostable multivibrator in turn is connected by diodes, such as diode 289, to each of the reset stages of the multivibrators 276-280.

Multivibrator 126 includes a control circuit comprised of a condenser 290 and a resistor 291 which are adjusted so that multivibrator 126 has a period of three seconds.

Multivibrator 287 each actuation of the valve 131 sufficient to permit the hydraulic system for valve 131 to recover so that it can be actuated upon the next succeeding pulse which appears after the expiration of the eight second delay period.

Briefly summarized, the system of FIGURE 9 operates as follows. The output from the receiver 40 is fed through three emitter-followers 205-207 and is then stepped up voltage-wise by a factor the rectifying diode 230.

In order to convert the DC. voltage to a frequency, the integrator-level detector units are employed, utilizing two high-gain operational parent resistance of the formation.

In a particular circuit of the form illustrated, the output of integrator 243 varies from approximately +16 vided down at point 250 to produce the voltage applied to the a input of differential amplifier 242. Since E tends to be maintained at a common voltage of 8 volts,

E242 where:

t is in seconds.

The voltage applied to the terminal a of the differential amplifier 242 is:

10 Since the differential amplifiers will switch when the two voltages are equal, the solution for t in terms of Rb is:

2.25 Rb Since the integrator 243 is reset almost instantly and since the output pulses are applied to the divider 125, a final outputv pulse period applied to the control solenoid is:

E Ra Since depletion of the valve is to be avoided, an 8 second delay is incorporated which inhibits the divider 125 so that the final pulse period received at the surface is:

72 T m 8 5 The surface equipment upon receipt of each pulse shuts down for a period of 8 seconds. Thus, there is obtained a frequency at the surface which is:

The transmitting toroid 50 was formed with of turns of No. 30

40 including a Winding a winding copper wire. The receiving toroid seconds in the embodiment described herein) is provided for recording a pulse train which is representative of formation resistivity.

In FIGURE 6 the response of the surface detector 300 of FIGURE 1 has been illustrated. The surface mud pressure is plotted along the ordinate 301 and time plotted as abcissa 302. The relatively long period as between pulses 303 and 304 may represent low resistivity. The shorter interval as between pulses 305 and 306 may represent high resistivity. In practice, the pressure pulses appear distinctly above the noise in the mud stream as produced by the pump. They have been found to be clearly discernible above the noise and provide an accurate measure of resistivity.

The signal appearing at terminals 100 of FIGURE 5 which is proportional to the excitation of the transmitter 50 may be cou led to the signal appearing at the output terminals 113 of FIGURE 5 in a circuit which provides an output which is proportional to the ratio of the two signals. The output signal applied to the preamplifier 121 would then be proportional to the ratio of the input and output signals and would thus provide for compensation for variations in the system due to changes in temperature and the like.

Thus, the present invention involves a system for logging a well while drilling in which a drill string terminates in a sub having a bit at the end thereof. A transmitting toroidal coil encompasses the sub preferably at a point spaced above the bit. An elecrical source is connected to the coil for applying alternating current preferably at a predetermined level to produce an alternating magnetic flux. Alternating current is thus induced for flow through the drill sub and the bit. A receiver toroidal coil encompasses the sub immediately adjacent to the bit for detecting current flow to the formation from the bit. Preferably any wall portion of the sub below the receiver is covered with an insulating material so that current coupled to the receiver will flow to the formation through the bit. A signaling means is provided which is connected to the detecting coil for generating a physical condition detectable at the mouth of the well.

In one aspect of the invention, the condition is dependent upon the ratio of the reference voltage at the input to the transmitter and the output voltage at the terminals of the receiver. A system for obtaining such a ratio may be in accordance with well-known principles such as disclosed in the Handbook of Automation Computation and Control by Grabbe et al., John Wiley & Sons, Inc., 1959, at page 23-12, et seq. In a further aspect of the invention, the transmitter and receiver are mounted on the outer wall of the sub and are embedded in an insulating body. A protective metallic ring is provided for each of the transmitter and receiver to protect them against abrasion. The rings are so mounted as to prevent short circuits around the transmitter or receiver. In a preferred embodiment, exposed metallic walls in a sub between the transmitter and receiver permit current flow from the sub to adjacent formations to form a guard current which requires flow of bit current at substantial depths within the formation beyond the borehole walls.

Having described the invention in connection with certain specific embodiments thereof, it is to be understood that further modifications may now suggest themselves to those skilled in the art and it is intended to cover such modifictaions as fall within the scope of the appended claims.

What is claimed is:

1. In a Well drilling system having means in a drill sub and bit at the bottom of said sub and a pressure pulse generator in the stream of drilling mud flowing to said bit, the combination which comprises:

(a) a transmitting toroid mounted in the outer wall of said sub at a point spaced above said bit,

(b) a receiving toroid mounted in the outer wall of said sub immediately adjacent to said bit,

(c) an alternating current source mounted in said sub' connected to said transmitting toroid for applying alternating current at a predetermined level thereto to induce alternating current flow longitudinally through said sub and said bit at the bit-formation contact, (d) means housed within said and into the formation sub for translating the voltage induced in said receiving toroid by said alternating current flow into time modulated electrical pulses, and

(e) means for applying said cal pulses mission of time modulated time modulated electrito said pressure pulse generator for transpressure pulses to the earths surface through said stream which are representative of the resistivity of contact thereof with said bit.

the formations at the 2. A resistivity logging system which comprises the combination of z (a) a drill collar adapted to receive a drill bit at the lower end thereof for contact with bottom hole formations,

(b) a first toroid encircling said drill collar immediately adjacent to said bit, (0) a second toroid encircling point spaced from said first (d) means for energizing said said drill collar at a toroid,

second toroid with alternating current at a predetermined level for the production of current flow through said drill collar said first toroid and said bit into said formations at the bit-formation contact,

(e) generating means in said drill collar for producing pressure pulsations in the drilling fluid flowing to said formation through said bit for transmission to the earths surface, and (f) means coupled to said said generating magnitude of said current flowing formation contact.

first toroid for energizing means at a rate proportional to the through said bit- 3. A system for logging a well while drilling which comprises:

(a) a drill collar having a bit at the end thereof,

(b) a toroidal a point spaced above said bit, (0) an electrical source means for applying alternating determined level to produce coil encompassing said drill collar at connected to said coil current thereto at a premagnetic flux within said coil to induce electrical current flow through said drill collar to said bit,

(d) a detector coil encompassing said drill collar immediately adjacent to said bit for formations of varying resistivity flow from said bit to detecting current exposed and contacted by said bit, (e) signaling means connected to said detector coil for translating voltages induced therein by said current flow into physical conditions for transmission to month of said well, and

mouth for sensing and conditions as a function of the depth for logging formation resistivity (a) an elongated sub adapted to receive a drill bit at the lower end thereof and to be coupled to a drill stem at the upper end thereof, said sub having a large upper flow channel extending to a small lower flow channel, (b) an instrument housing secured in said upper flow channel with an annular zone between said housing and the inner walls of said upper flow channel for annular flow of drilling fluid past said housing,

(0) a flow crossover means having an isolation channel therein and located in the lower portion of said housing for directing flow channel, (d) a transmitter including a tap the annular flow into said lower e-wound magnetic core a nonconductive sub at a predetermined point and a toroidal winding encased in body encircling said spaced above said bit,

(e) a receiver including a tape-wound magnetic core and a toroidal winding encased in a nonconductive body encircling said sub immediately adjacent to said bit,

(f) oscillation means in said housing connected to said transmitter for generating alternating current in said sub and said bit, and

(g) means in said housing connected to said receiver through said crossover means for producing pressure pulses in said fluid above said housing at a rate dependent upon said resistivity.

5. An instrument sub for logging formation resistivity While drilling which comprises:

(a) an elongated sub adapted to receive a drill bit at the lower end thereof and to be coupled to a drill stem at the upper end thereof, said sub having a large upper flow channel extending to a small lower flow channel,

(b) an instrument housing secured in said upper flow channel therein and located in the lower portion of said housing for directing the annular flow into said lower flow channel,

spaced above said bit,

(e) a receiver including a tape-wound magnetic core and a toroidal winding encased in a nonconductive body encircling said sub immediately adjacent to said bit,

(f) means in said housing connected through said crossover means to said transmitter to energize said transmitter with alternating current,

while drilling which comprises:

(a) an elongated heavy-walled tube a bit at the lower end thereof,

(b) a transmitter having a tape-wound magnetic core with a toroidal winding mounted in the exterior Wall of said tube in an insulating body,

() an outer abrasion protective metallic band covering said transmitter,

(d) an insulating band mounted on said tube below said transmitter,

(e) a conductive cylindrical spacer mounted on said tube below said insulating body,

(f) a receiver including a tape-wound magnetic core with a toroidal winding mounted on said tube in an insulating body below said spacer, and

(g) an outer abrasion protective metallic band covering said receiver whereby,

adapted to receive tact for inducing in said receiver a voltage representative of resistivity of the formation, the combination which comprises: (a) valve means in the mud stream adjacent to said (b) a solenoid coupled to said valve means responsive to said signal normally to actuate actuation thereof, and (d) means at the mouth 0 pressure variations in spacer to avoid an electrical short circuit around said transmitter,

(f) a receiver located lower end of said sub including a magnetic core with below said spacer, and an outer abrasion protective metallic band covering said receiver the formation at the bit-formation contact for inducing in said receiver a voltage representative of resistivity of the formation at said contact. 11. A system for logging a well while drilling, where a thereof, which comprises:

point spaced above said bit,

(b) an electrical source means connected to said coil for applying alternating current thereto at a predetermined level to produce magnetic flux within said coil to induce electrical current flow through said drill collar to said bit,

(c) a detector coil encompassing said drill collar immediately adjacent to said bit for detecting current flow from said bit to formations of varying resistivity exposed and contacted by said bit,

(d) conversion means responsive to the voltage in said detector coil for producing a train of output pulses which are spaced one from another in dependence upon the magnitude of said voltage,

(e) signaling means for translating said output pulses into pressure pulses in the fluid in said well as to be detectable at the mouth of said well, and

(f) means for sensing and recording said conditions as a function of the depth of said bit in said well.

12. The combination set forth in claim 11 in which abrasion resistant shield means ensbrouds both said toroidal coil and said detector coil.

13. The combination set forth in claim 11 in which an abrasion resistant shield ring enshrouds said toroidal coil 20 but is mounted to prevent flow of current therethrough in any path that does not extend beyond the walls of said sub.

14. The combination set forth in claim 11 in which said conversion means produces said output pulses at variable time intervals which normally are shorter than the recovery time of said signaling means, and wherein means are provided for inhibiting actuation of said signaling means by said output pulses for a fixed period of time following each actuation thereof at least as great as the recovery time of said signaling means.

References Cited UNITED STATES PATENTS 2,389,241 11/1945 Silverman 324-1 XR 2,411,696 11/1946 Silverman et al. 324- 1 XR 2,524,031 10/1950 Arps 3241 2,700,131 1/1955 Otis et al. 324-1 2,759,143 8/1956 Arps 324-1 2,901,685 8/1956 Adler 324-1 XR 2,917,704 12/1959 Arps 324-1 3,094,658 6/1963 Bravenec et al. 324-6 RUDOLPH V. ROLINEC, Primary Examiner. G. R. STRECKER, Assistant Examiner. 

2. A RESISTIVITY LOGGING SYSTEM WHICH COMPRISES THE COMBINATION OF: (A) A DRILL COLLAR ADAPTED TO RECEIVE A DRILL BIT AT THE LOWER END THEREOF FOR CONTACT WITH BOTTOM HOLE FORMATIONS, (B) A FIRST TOROID ENCIRCLING SAID DRILL COLLAR IMMEDIATELY ADJACENT TO SAID BIT, (C) A SECOND TOROID ENCIRCLING SAID DRILL COLLAR AT A POINT SPACED FROM SAID FIRST TOROID, (D) MEANS FOR ENERGIZING SAID SECOND TOROID WITH ALTERNATING CURRENT AT A PREDETERMINED LEVEL FOR THE PRODUCTION OF CURRENT FLOW THROUGH SAID DRILL COLLAR SAID FIRST TOROID AND SAID BIT INTO SAID FORMATIONS AT THE BIT-FORMATION CONTACT, (E) GENERATING MEANS IN SAID DRILL COLLAR FOR PRODUCING PRESSURE PULSATIONS IN THE DRILLING FLUID FLOWING TO SAID FORMATION THROUGH SAID BIT FOR TRANSMISSION TO THE EARTH''S SURFACE, AND (F) MEANS COUPLED TO SAID FIRST TOROID FOR ENERGIZING SAID GENERATING MEANS AT A RATE PROPORTIONAL TO THE MAGNITUDE OF SAID CURRENT FLOWING THROUGH SAID BITFORMATION CONTACT. 